Pressure steam ageing of silica filled silicone rubber : Degradation mechanisms

• Silicone rubber exposed to alternating vaccum and dry saturated steam ageing cycles. • Silicone matrix undergo hydrolytic cleavage, recombination and backbiting reactions. • Significant changes at silica-polymer interfaces during initial ageing cycles. • The fraction of low-mobility dimethylsiloxy units on the silica surface increases. Silica filled silicone rubber compound was hydrothermally aged under a complex condition consisting of a succession of vacuum phases at 80 °C and exposure to pressure steam at 134 °C. The results from various physico-chemical analysis techniques indicate that hydrothermal ageing has significantly affected both the polymer network and the polymer-filler interfaces, allowing the identification of the primary degradation mechanisms. Solvent swelling and solid 29 Si CP-MAS NMR analysis evinced that hydrolytic scission reactions predominate in the early stages of ageing and are compensated by recombination reactions for the subsequent cycles. The volatilization of cyclic oligomers, resulting from the backbiting reaction, led to polymer loss and an associated increase in silica content within the silicone rubber compound, as confirmed by FTIR and TGA analyses. Significant changes occurred at the polymer-filler interfaces during the first few hundred cycles, including the formation of covalent bonds at the silica surface, replacing the physical interactions. These findings were further supported by solid-state 29 Si CP-MAS NMR analysis and ammonia-modified swelling experiments.